pico-imx7d: add config to enable CAAM

Add config to enable CAAM so that hardware random number generator
works.

Signed-off-by: Jun Nie <jun.nie@linaro.org>
diff --git a/configs/pico-imx7d_bl33_defconfig b/configs/pico-imx7d_bl33_defconfig
index d775244..c3c309c 100644
--- a/configs/pico-imx7d_bl33_defconfig
+++ b/configs/pico-imx7d_bl33_defconfig
@@ -4,6 +4,7 @@
 CONFIG_SPL_GPIO_SUPPORT=y
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
+CONFIG_SECURE_BOOT=y
 CONFIG_TARGET_PICO_IMX7D=y
 CONFIG_SPL_MMC_SUPPORT=y
 CONFIG_SPL_SERIAL_SUPPORT=y